Human T-cell growth factor (TCGF, interleukin-2) is a 13,000-molecular weight protein secreted into culture media by PHA-stimulated lymphocytes. This lymphokine has a selective proliferative effect on certain malignant T cells and lectin-\or antigen-activated normal T cells, but no effect on resting T lymphocytes. In order to gain insight into the mechanism of its selective mitogenic effect, purified human TCGF labeled with iodine-125 or activated biotin will be employed to determine the spectrum of cells with membrane receptors for TCGF. Binding studies with labeled TCGF will involve a survey of normal and malignant hematopoietic cells, with an emphasis on cutaneous T-cell lymphomas and T-cell leukemias. In addition, the suppressive effects of the conditioned media of an OKT8-positive human leukemia cell line on Il-2-dependent T-cell proliferation will be examined in terms of the ability of the suppressive factor to block the binding of Il-2 to activated T cells. Biotin-TCGF will be used with fluorescein-avidin in the fluorescence-activated cell sorter to correlate the presence of TCGF receptors with cell size, DNA content and the presence of other surface markers such as Ia antigens, peanut agglutinin receptors and surface antigens recognized by monoclonal antibodies directed against specific T-cell subsets. The blocking of the binding of labeled TCGF to activated T cells will be used as a screening assay for the production of monoclonal antibodies against the Il-2 receptor. Similarly, monoclonal antibodies against the suppressive factor will be generated by using the capacity of the antisera to block the suppressive effect of the factor on Il-2-dependent T cells as a screening assay.